Microelectronic fluid detector

ABSTRACT

A resistive microelectronic fluid sensor implemented as an integrated voltage divider circuit can sense the presence of a fluid within a fluid reservoir, identify the fluid, and monitor fluid temperature or volume. Such a sensor has biomedical, industrial, and consumer product applications. After fluid detection, the fluid can be expelled from the reservoir and replenished with a fresh supply of fluid. A depression at the bottom of the sample reservoir allows a residual fluid to remain undetected so as not to skew the measurements. Electrodes can sense variations in the resistivity of the fluid, indicating a change in the fluid chemical composition, volume, or temperature. Such fluctuations that can be electrically sensed by the voltage divider circuit can be used as a thermal actuator to trigger ejection of all or part of the fluid sample.

BACKGROUND Technical Field

The present disclosure relates to microelectronic fluid sensors on boardintegrated circuit chips.

Description of the Related Art

Fluids can be integrated with microelectronic circuitry in manyapplications including, for example, biomedical devices configured toperform physiological tests on samples of bodily fluids, and thermalactuators that eject fluid in response to changes in temperature.

Approximately 25-30 million people in the United States are diagnosedwith type II diabetes requiring monitoring of blood glucose at leasttwice daily. A common example of a biosensor device that relies on amicroelectronic fluid sensor is a disposable test strip used formeasurement of blood glucose levels in diabetic patients as shown inFIG. 1. A typical blood glucose monitoring apparatus 80 includes aconventional disposable strip biosensor 82 and a portable electronicmonitor 83. The biosensor 82 is made of a semi-rigid backing material 84approximately an inch long, impregnated with an electrolytic chemicalreagent 86 at one end and printed with electrodes 88 at the other end.The patient pricks a fingertip, applies a blood sample 89 to theelectrolytic chemical reagent 86, and inserts the electrodes 88 into theportable electronic monitor 83. The electrolytic chemical reagent 86conducts a current that is proportional to an amount of glucose in theblood sample 89. Current flow conducted via the electrodes 88 in thebiosensor 82 closes a circuit when the biosensor 82 is inserted into theportable electronic monitor 83. The current in the circuit can then bemeasured by the portable electronic monitor 83. The portable electronicmonitor 83 is configured with software that converts the currentmeasurement into a numerical value that represents the blood glucoselevel. The portable electronic monitor 83 then provides a digitalreadout of the numerical value and stores the numerical value as bloodglucose data in an electronic memory. By either recording or downloadingthe blood glucose data, the patient can track blood glucose values overtime to adjust insulin dosage.

Use of an impregnated biosensor strip is problematic for severalreasons. The chemical reagent 86 may degrade over time such that thebiosensor strip has a finite shelf life and must be stamped with anexpiration date. In addition, this type of biosensor strip is expensive,and available only on a prescription basis, as opposed to being an itemthat is sold over-the-counter. When the liquid sample is applied,sometimes the strip fails to take up enough of the liquid volume to makean accurate reading, and the test must be repeated, which incurs evenmore expense. Finally, the strips are disposable and cannot be re-used.

Another type of fluid detector that can be used to detect electricalproperties of a fluid sample such as the blood sample 89, uses an openfluid reservoir instead of an impregnated reagent. An example of such adetector is a capacitive fluid detector as shown and described belowwith respect to FIGS. 2A, 3A, and 3B. Such a capacitive fluid detectortransmits electrical signals through the fluid sample in the reservoir.The electrical signals can be compared against previous signals or anindependent standard to detect changes. Changes in the electricalsignals can indicate the presence or absence of fluid in a regionbetween two electrodes, for example. Once presence of the fluid sampleis detected, further changes in such signals can indicate fluctuatinglevels of fluid components that are charge-dependent such as glucose,electrolytes, or ions such as calcium, magnesium, potassium, and thelike. The electrical signal data can then be sent to a microprocessor tocalculate corresponding electrical properties of the fluid sample.

Depending on the design of the sensor, such a capacitive detectionsystem may provide information regarding the presence of fluid, or thepresence of certain components within the fluid, but not necessarilyinformation regarding an amount of fluid present. For example, if thefluid participates in the circuit as part of a capacitive electroderather than part of a capacitive dielectric, the capacitor geometry maynot allow distinguishing between a small volume of fluid and a largevolume. In the parallel plate capacitor sensor described above, thefluid is typically incorporated as a portion of one of the electrodes.However, where the fluid is incorporated as the dielectric, or a portionof the dielectric, it becomes possible to identify the fluid based onthe dielectric constant. Such an arrangement is not feasible in the caseof a capacitive sensor, however, because the dielectric, beingsandwiched between two metal plates, is not easily accessible forintroduction of a fluid sample by a user. Furthermore, capacitive sensormeasurements may be affected by parasitic capacitances elsewhere in thecircuit that are not actually related to the fluid sample and cantherefore skew the test results. For at least these reasons, it may bedesirable to have other types of fluid sensors available on anintegrated circuit chip in addition to, or in place of, capacitive fluidsensors.

BRIEF SUMMARY

A resistive microelectronic fluid detection system can determine moreinformation about a fluid than a typical capacitive fluid detector can,and with greater accuracy, in part because the resistive fluid detectoris not subject to parasitic effects. In addition to sensing the presenceof a fluid in a reservoir, a resistive microelectronic fluid sensor canidentify the fluid and determine its volume.

Embodiments of a resistive microelectronic fluid detector can be builtso as to include a fluid reservoir that is at least partially exposed.An electrical signal can then be applied laterally across the fluidreservoir. Presence or absence of a fluid can then be determineddirectly and with less influence from external factors, for example, byapplying a voltage and measuring whether or not a current flows throughthe sample reservoir. If a current flows and closes the circuit, it canbe deduced that a sample is present. In one embodiment of a resistivemicroelectronic fluid detector, the bottom of the sample reservoir ismodified with a depression so that a certain volume of fluid can bepresent without conducting the electrical signal. Using such a modifiedreservoir, when a signal is detected, it is known that the volume of thesample is above a certain threshold value.

In another embodiment, if a second resistor is constructed adjacent tothe fluid reservoir, simple equations for a voltage divider circuitallow determination of the fluid sample volume if the fluid resistivityis known. Conversely, identification of the fluid is possible if thevolume of the sample is known.

In another embodiment, the sample reservoir can be extended verticallyand capped to form a partially or substantially enclosed chamber withina microfluidic ejection system. In such a system, detection of athreshold volume of fluid within the reservoir can trigger ejection ofthe fluid sample out of the chamber, through a nozzle in the cap.Electrodes contacting the fluid within the reservoir can further be usedto sense differences or variation in the resistivity of the fluid,indicating a different type of fluid is present, or a change in thefluid chemical composition or the fluid volume. In some applications,for example, changes in resistivity of the fluid can indicatetemperature fluctuations. Such fluid characteristics that can beelectrically sensed by a voltage divider circuit can in turn be used asa thermal actuator to trigger ejection of all or part of the fluidsample.

The fluid detector described herein may be used in conjunction with auniversal flexible micro-sensor as described in U.S. Patent PublicationNo. 2015/0253276, by the same inventors as the present patentapplication.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements.The sizes and relative positions of elements in the drawings are notnecessarily drawn to scale.

FIG. 1 is a pictorial view of a typical blood glucose monitoring systemthat employs a test strip impregnated with an electrolytic chemicalreagent, as found in the prior art.

FIG. 2A is a schematic diagram of a circuit that includes a conventionalcapacitive fluid sensor, according to the prior art.

FIG. 2B is a schematic of a resistive microelectronic fluid detector, asdescribed herein, according to one embodiment.

FIG. 3 is a cross-sectional view of an integrated circuit implementationof a conventional capacitive fluid sensor.

FIG. 4A is a schematic of a resistive microelectronic fluid detector inthe form of a voltage divider circuit element that includes a deviceunder test and a known reference device.

FIG. 4B is a top perspective view of a first integrated circuitembodiment of the microelectronic fluid detector shown in FIG. 4A.

FIG. 4C is a cross-sectional view of a first integrated circuitembodiment of the microelectronic fluid detector shown in FIG. 4B, takenalong the vertical cut line A-A′ through the center of the fluidreservoir.

FIG. 5 is a flow diagram of a method of making the first integratedcircuit microelectronic fluid detector shown in FIGS. 4B and 4C.

FIG. 6A reproduces the schematic diagram of the voltage divider circuitelement shown in FIG. 4A.

FIG. 6B is a top perspective view of a second integrated circuitembodiment of the microelectronic fluid detector shown in FIG. 6A.

FIG. 6C is a cross-sectional view of a second integrated circuitembodiment of the microelectronic fluid detector shown in FIG. 6B, takenalong the vertical cut line A-A′ through the center of the fluidreservoir.

FIG. 7 is a flow diagram of a method of making the second integratedcircuit microelectronic fluid detector shown in FIGS. 6B and 6C.

FIG. 8 is a block diagram of a microelectronic fluid sensor system asdescribed herein.

FIG. 9 is a flow diagram of a method of operating an integrated circuitmicroelectronic fluid detector to identify a fluid.

FIG. 10 is a flow diagram of a method of operating an integrated circuitmicroelectronic fluid detector to determine a fluid volume.

FIG. 11A is a top perspective view of an integrated circuit embodimentof a microfluidic ejection system as described herein.

FIG. 11B is a cross-sectional view of the microfluidic ejection systemshown in FIG. 9A, taken along the vertical cut line A-A′.

FIG. 12 is a flow diagram showing three alternative methods of operationof the microfluidic ejection system shown in FIGS. 11A and 11B.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various aspects of thedisclosed subject matter. However, the disclosed subject matter may bepracticed without these specific details. In some instances, well-knownstructures and methods of semiconductor processing comprisingembodiments of the subject matter disclosed herein have not beendescribed in detail to avoid obscuring the descriptions of other aspectsof the present disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thespecification are not necessarily all referring to the same aspect.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more aspects of the presentdisclosure.

Reference throughout the specification to integrated circuits isgenerally intended to include integrated circuit components built onsemiconducting substrates, whether or not the components are coupledtogether into a circuit or able to be interconnected. Throughout thespecification, the term “layer” is used in its broadest sense to includea thin film, a cap, or the like and one layer may be composed ofmultiple sub-layers.

Reference throughout the specification to conventional thin filmdeposition techniques for depositing silicon nitride, silicon dioxide,metals, or similar materials include such processes as chemical vapordeposition (CVD), low-pressure chemical vapor deposition (LPCVD), metalorganic chemical vapor deposition (MOCVD), plasma-enhanced chemicalvapor deposition (PECVD), plasma vapor deposition (PVD), atomic layerdeposition (ALD), molecular beam epitaxy (MBE), electroplating,electro-less plating, and the like. Specific embodiments are describedherein with reference to examples of such processes. However, thepresent disclosure and the reference to certain deposition techniquesshould not be limited to those described. For example, in somecircumstances, a description that references CVD may alternatively bedone using PVD, or a description that specifies electroplating mayalternatively be accomplished using electro-less plating. Furthermore,reference to conventional techniques of thin film formation may includegrowing a film in situ. For example, in some embodiments, controlledgrowth of an oxide to a desired thickness can be achieved by exposing asilicon surface to oxygen gas or to moisture in a heated chamber.

Reference throughout the specification to conventional photolithographytechniques, known in the art of semiconductor fabrication for patterningvarious thin films, includes a spin-expose-develop process sequencetypically followed by an etch process. Alternatively or additionally,photoresist can also be used to pattern a hard mask (e.g., a siliconnitride hard mask), which, in turn, can be used to pattern an underlyingfilm.

Reference throughout the specification to conventional etchingtechniques known in the art of semiconductor fabrication for selectiveremoval of polysilicon, silicon nitride, silicon dioxide, metals,photoresist, polyimide, or similar materials includes such processes aswet chemical etching, reactive ion (plasma) etching (RIE), washing, wetcleaning, pre-cleaning, spray cleaning, chemical-mechanicalplanarization (CMP) and the like. Specific embodiments are describedherein with reference to examples of such processes. However, thepresent disclosure and the reference to certain deposition techniquesshould not be limited to those described. In some instances, two suchtechniques may be interchangeable. For example, stripping photoresistmay entail immersing a sample in a wet chemical bath or, alternatively,spraying wet chemicals directly onto the sample.

Specific embodiments are described herein with reference tomicroelectronic fluid sensors and fluid ejection devices that have beenproduced; however, the present disclosure and the reference to certainmaterials, dimensions, and the details and ordering of processing stepsare exemplary and should not be limited to those shown.

FIG. 2A shows a fluid detection circuit 100 that provides context for aconventional capacitive fluid sensor 102, according to the prior art.When nodes 1 and 2 are energized, a transistor 103 turns on and appliesa signal to the conventional capacitive fluid sensor 102. Thecapacitance value is sensed and this provides an indication of the fluidbeing tested.

FIG. 2B shows a resistive microelectronic fluid detector 104, asdescribed herein, that is proposed to replace the conventionalcapacitive fluid sensor 102, for example, adjacent to nodes 1 and 2 inthe circuit 100. In one embodiment, the resistive microelectronic fluiddetector 104 is in the form of a voltage divider that includes a deviceunder test (DUT) 106 coupled in parallel to a known reference device 108having a resistance R that is much larger than the resistance r of theDUT 106. For example, R can be on the order of 10-100 kΩ, while r canvary over a range of a very high value (several GΩ or higher) when thefluid under test is not present, to a lower value, such as approximately1 kΩ when the fluid under test is present.

FIG. 3 shows an integrated circuit embodiment of the conventionalcapacitive fluid sensor 102. The integrated capacitive fluid sensor 102is in the form of a parallel plate capacitor that includes a bottomelectrode 112, a dielectric 114, and a top electrode 116. The bottom andtop electrodes 112 and 116 are generally metallic while the dielectric114 can be made of a passivation material or another dielectricavailable in a semiconductor fabrication process such as an inter-layerdielectric (ILD) material. The top electrode 116 includes a fluidreservoir 118 intended to contain a volume of electrolytic fluid undertest, for example, a blood sample, a DNA sample, an industrial chemicalsample, or the like. The electrical conductivity of the fluid sample inthe fluid reservoir 118 is thus exploited to engage the fluid sample asan integral part of the top electrode 116 of the parallel platecapacitor. If the fluid reservoir 118 is empty, the capacitance of theparallel plate capacitor will be reduced proportionally, according tothe relationship C=KA/d, wherein A is the surface area of the topelectrode 116, K is the dielectric constant of the dielectric 114, and dis the thickness of the dielectric 114. When the fluid reservoir 118 isempty, the electrode surface area A is reduced. Therefore, the presenceor absence of fluid in the fluid reservoir 118 is detectable by theconventional integrated capacitive fluid sensor 102. However,information regarding an amount of fluid present in the fluid reservoir118 is not provided by such a conventional capacitive sensor.

A microelectronic fluid detector 104, as described herein, is realizedas a miniature integrated voltage divider circuit that is proposed as asubstitute sensor to use in place of the conventional integratedcapacitive fluid sensor 102. The integrated circuit voltage dividerincludes a known reference element (R) in the form of a resistive thinfilm layer, and an unknown resistance in the form of a fluid reservoir126 (r), positioned between two electrodes. The resistance r varies withchanges in the fluid present in the reservoir 126.

Two embodiments of such an integrated circuit voltage divider, 104 a and104 b, are presented herein in FIGS. 4B and 4C, and FIGS. 6B and 6C,respectively. FIG. 4A reproduces the schematic representation of thevoltage divider. FIG. 4A is shown for comparison against the top planview shown in FIG. 4B and the cross-sectional view shown in FIG. 4C.During operation of the resistive microelectronic fluid detector 104, anelectrical signal is applied at electrodes 120 and 122 to test a fluidsample, which is the device under test 106.

FIGS. 4B and 4C show different views of the first embodiment 104 a ofthe microelectronic fluid detector 104. With reference to FIGS. 4B and4C, the first embodiment 104 a is one integrated circuit implementationof the resistive microelectronic fluid detector 104. The firstembodiment 104 a of the voltage divider circuit 104 includes a highresistance material 131 as the reference device (R) and a fluidreservoir 126 as the resistor r. The two resistors are coupled byelectrodes 123 a and 123 b, and by vias 120 and 122 through a firstinsulator 133. A second insulator 124 separates the electrodes 123 a and123 b so that they are laterally spaced apart from one another, and canbe coupled through the fluid in the reservoir 126. The first embodiment104 a can be built on a semiconductor substrate 119, according to stepsin an exemplary method 130, as shown in FIG. 5. Alternatively, thesubstrate 119 can be an insulator, such as a polymer layer, a polyimidelayer, a glass layer, or the like.

At 132, a high resistance material 131 that serves as the referencedevice 108 is formed overlying the substrate 119. In one embodiment, thehigh resistance material 131 is a high resistance conductor such assilicon carbide (SiC). The high resistance material 131 could also be alightly doped polysilicon or other high resistance material. In otherembodiments, the material serving as the reference device 108 can be apassivation material such as a doped or conductive polyimide that can bespun onto the substrate in liquid form. Alternatively, another insulatorwith some conductive properties can be deposited using a conventionalthin film deposition process.

At 134, the first insulator 133 is formed on top of the high resistancematerial 131 by depositing an inter-layer dielectric (ILD) material suchas, for example, silicon dioxide (SiO₂) using a conventional thin filmdeposition process.

At 136, vias are patterned in the first insulator 133 using aconventional photolithography and etching process sequence.

At 138, the vias are filled with metal to form coupling wires 120 and122.

At 140, a metal layer is deposited on top of the first insulator 133.The metal layer can be made of copper, silver, platinum, gold, titanium,tungsten, and the like, or alloys thereof.

At 142, a first portion of the metal layer is removed and filled with asecond insulator 124 as shown in FIG. 4B, thus creating electrodes 123 aand 123 b. The second insulator 124 can be made from an oxide or nitridematerial, for example. If the height of the second insulator 124, asdeposited, exceeds that of the electrodes, the top surface of the devicecan be planarized by polishing the insulator material and stopping onthe surface of the electrodes. Otherwise, the electrodes can be polishedto stop on the second insulator 124. The second insulator 124 ensuresthat the electrodes 123 a, 123 b remain spaced apart from one another sothat an applied signal will be conducted through the fluid sample as thedevice under test 106, and not be short-circuited between the electrodes123 a, 123 b.

At 144, a second portion of the metal layer is removed, for example byetching, to create a fluid reservoir 126. Thus, the electrodes 123 a,123 b and the second insulator 124 together form walls that bound thefluid reservoir 126 on four sides, while the first insulator 133 forms afloor that bounds the fluid reservoir 126 from below. The reservoir 126can extend below the dielectric-metal interface, into the firstinsulator 133 in one embodiment, but this is not required.

The second embodiment 104 b of the voltage divider circuit 104 alsoincludes a high resistance material 131 as the reference device (R) anda fluid reservoir 126 as the resistor r. The two resistors are coupledby electrodes 120 and 122 which are formed with lower portions 120 a and122 a on either side of the reference device R and upper portions 120 band 122 b on either side of the fluid reservoir 126 (r). The secondinsulator separates the upper portions of the electrodes 120 b and 122 bso that they are laterally spaced apart from one another, and can becoupled through the fluid in the reservoir 126.

FIG. 6A reproduces the schematic representation of the resistivemicroelectronic fluid detector 104 in the form of a voltage divider thatis proposed as a substitute sensor to use in place of the conventionalintegrated capacitive fluid sensor 102. FIG. 6A is provided forcomparison against the top plan view shown in FIG. 6B and across-sectional view shown in FIG. 6C of a second embodiment 104 b ofthe resistive microelectronic fluid detector 104. The second embodiment104 b can be built on the substrate 119, according to a method 150, asshown in FIG. 7.

At 152, a high resistance material 131 that serves as the referencedevice 108 is formed on top of the substrate 119, using a conventionalthin film deposition process. In one embodiment, the reference device108 has such a high resistance R, it conducts little to no current and,thus, could be considered an open circuit. In other embodiments, thereference device 108 is a high resistance conductor, such as SiC orlightly doped polysilicon.

At 154, a first insulator 133 is formed in contact with the highresistance material 131.

At 156, trenches are patterned in the first insulator 133 using aconventional photolithography and etching process sequence, to create aninsulator block that provides vertical separation between the deviceunder test 106 from the reference device 108.

At 158, the trenches are filled with metal to form lower portions of theelectrodes, 120 a and 122 a, shown in FIG. 6C.

At 160, an additional metal layer is deposited on top of the filledtrenches which will form upper portions of the electrodes 120 b and 122b, shown in FIG. 6C.

At 162, a first portion of the additional metal layer is removed andfilled with a second insulator 124. The second insulator 124 can be madefrom the same material as the first insulator 133, or it can be made ofa different material. If the height of the second insulator 124, asdeposited, exceeds that of the electrodes, the top surface of the devicecan be planarized by polishing the second insulator 124 and stopping onthe upper portions of the electrodes 120 b, 122 b. Otherwise, theelectrodes can be polished to stop on the second insulator 124. Thesecond insulator 124 ensures that the electrodes 120 b, 122 b remainspaced apart from one another so that an applied signal will beconducted through the fluid sample as the device under test 106, and notbe short-circuited between the electrodes.

At 164, a second portion of the metal layer is removed to create thefluid reservoir 126. Thus, the electrodes 120 b, 122 b and the secondinsulator 124 together form walls that bound the fluid reservoir 126 onfour sides, while the high resistance material 131 forms a floor thatbounds the fluid reservoir 126 from below.

FIG. 8 shows a microelectronic fluid sensor system 166 including amicroprocessor 168 and an electronic memory 169, both of which can belocated on the same integrated circuit chip as the microelectronic fluiddetector 104 that includes the device under test 106 and the referencedevice 108. The electronic memory 169 stores instructions for executionby the microprocessor 168 to test fluid samples. In addition, theelectronic memory 169 stores other data such as, for example, materialinformation including resistivity values, material constants, and thelike to support calculations such as those described herein.

There are two alternative embodiments to use for the substrate 119 whenit takes the form of a semiconductor substrate. In a first embodiment,the semiconductor substrate 119 has, in addition to a foundation of asemiconductor substrate layer, electronic circuits formed therein. Suchelectronic circuits include transistors having source and drain regionsand electrical interconnections coupling them to form logic gates. Inthis embodiment, the microprocessor, memory and other electroniccircuits are formed in the semiconductor substrate 119, as well asnumerous other layers overlying the semiconductor substrate 119 itselfwhich are not specifically shown in FIGS. 4B and 4C. For example, therewill be numerous interconnect metal layers and insulating layers whichoverlay the transistors formed in the substrate itself. Accordingly,reference to the semiconductor substrate 119 includes such integratedcircuits as a whole, that is to say, the transistors and theinterconnect layers connecting them. The numerous layers making up thetransistors and the interconnect structure are omitted herein for easeof reference, and it is well understood by those of skill in art how tobuild an integrated circuit overlying a semiconductor substrate 119.

According to one embodiment, the high resistance material 131 will beone of the top most metal layers of the integrated circuit whichoverlays the semiconductor substrate 119. The various insulating andmetal interconnect layers which form the microprocessor are formed on adifferent part of the die than that section shown in FIG. 4C and, thus,are not shown for ease in illustration. In most embodiments, there willbe an insulating layer positioned between the conductive high resistancematerial 131 and the semiconductor substrate 119 itself and, as justexplained herein, there may be numerous alternating conductive andinsulating layers which electrically isolate the high resistancematerial 131 from the semiconductor substrate itself.

In an alternative embodiment, the substrate 119 is composed of a fullyinsulating material instead of being a semiconductor substrate. Forexample, the substrate 119 may be a polymer, a polyimide film or othersupporting substrate which is capable of having a high resistancematerial 131 formed thereon. One technique by which the sensor 104 canbe formed in which the substrate 119 is a polymer layer is described inU.S. patent application Ser. No. 14/200,828, incorporated herein byreference in its entirety.

FIG. 9 shows steps carried out by the microprocessor 168 in an exemplarymethod 170 of operating the microelectronic fluid detector 104,according to one embodiment.

At 171, the fluid reservoir 126 is tested by applying an electricalsignal to the voltage divider. For example, a voltage can be appliedbetween electrodes 120 and 122 by coupling a power source to theelectrodes 120 and 122.

At 172, a current is measured between the electrodes 120 and 122. If thefluid reservoir 126 is empty, all the current will flow through thereference device 108, according to the relationship

I=V _(applied) /R,  (1)

and no current will flow through the resistor r, which is the fluidreservoir 126. On the other hand, when the fluid reservoir 126 containsa fluid sample, the total electric current is divided so that somecurrent flows through each of the fluid reservoir 126 (r) and theinsulator block (R). The total current is thus given by

I=V _(applied)(1/R+1/r)  (2)

At 173, based on the measured value of I, it is determined by themicroprocessor whether or not a current is flowing through the fluidreservoir.

At 174, if a current is flowing through the reservoir, it is concludedthat a fluid is present in the fluid reservoir 126.

At 175, given that a fluid is present in the fluid reservoir 126, theresistance of the fluid sample, r, can be computed from equation (2)above, wherein V is applied, I is measured, and R is a known resistancethat can be computed from the known geometry and material parameters ofthe high resistance material 131.

At 176, since the geometry of the fluid reservoir 126 is known, aresistivity, ρ, of the fluid sample can be calculated according to thewell-known relationship

r=μL/A  (3)

wherein A is a surface area of the reservoir transverse to the directionof current flow, and L is the length of the reservoir. It is assumed, inthe present embodiment, that the fluid sample substantially fills thereservoir 126.

At 177, once the resistivity is known, a lookup table of variousmaterial parameters stored in the electronic memory 169 can be consultedto identify the type of fluid present in the fluid reservoir 126.

If the type of fluid is known, FIG. 10 shows steps carried out by themicroprocessor 168 in an exemplary method 180 of operating themicroelectronic fluid detector 104, to determine fluid volume of a knownfluid sample, according to one embodiment.

At 182, an electric current is applied to the microelectronic fluiddetector 104, wherein the fluid reservoir 126 contains a sample of anidentified fluid.

At 184, a voltage is measured across the fluid reservoir 126.

At 186, the resistance, r, of the fluid sample is computed from equation(2).

At 188, the volume of the fluid sample is computed as V=AL wherein A isknown and L is determined from equation (3).

FIGS. 11A and 11B show a top plan view and a cross-sectional view,respectively, of a microfluidic ejection system 190 according to oneembodiment as described herein. The microfluidic ejection system 190includes elements of the microelectronic fluid sensor 104 a describedabove, including the high resistance material 131 used as the referencedevice 108, the electrodes 120 and 122, the second insulator 124, andthe fluid reservoir 126.

In the microfluidic ejection system 190, the lower boundary of the fluidreservoir 126 further includes a depression 192 that allows a smallvolume of fluid 193 to be captured on the bottom of the reservoir whileremaining isolated from the electrodes 120 and 122. In addition, themicrofluidic ejection system 190 further includes an encapsulant 194 ontop of the electrodes, the encapsulant enclosing a chamber 196 locateddirectly above the reservoir 126. The chamber 196 is then substantiallyenclosed by a cap 198. In the embodiment shown in FIG. 10B, theencapsulant 194 and the cap 198 are both made of the same flexibleinsulating material, which can be a polymer or a dielectric.

The cap 198 includes a nozzle 200 substantially vertically aligned withthe depression 192, through which fluid in the reservoir 126 can beejected in response to a signal from a controller, for example, themicroprocessor 168. Following ejection of fluid, the reservoir 126 canbe refilled via microfluidic channels from a fluid supply external tothe microfluidic ejection system 190.

FIG. 12 illustrates an exemplary method 200 of operating themicrofluidic ejection system 190 that can be carried out by a controllersuch as the microprocessor 168, for example. The basic operation of themicrofluidic ejection system 190 as shown includes a probe to obtaininformation about the fluid in the reservoir, to determine whether ornot to eject the fluid and refill the reservoir, or to allow the fluidto remain in the reservoir.

At 202, a probe of the fluid in the reservoir 126 is activated todetermine information about the fluid sample using the resistive fluiddetector 104 within the microfluidic ejection system 190. For example,at 204, the type of fluid is verified; at 206, the fluid temperature issensed and evaluated; and at 208 the volume of fluid in the reservoir126 is determined and evaluated. Each of these determinations isdescribed in further detail below.

At 204, if the fluid in the reservoir 126 is not known, a fluidverification method can be executed to identify the fluid and decidewhether or not to clear the reservoir 126. First, the fluid isidentified using the method 160 described above.

At 210, it is determined whether or not the fluid present in thereservoir matches a desired fluid that is expected to be present.

At 212, if the desired type of fluid is present, the identificationmethod 160 can be repeated continuously or periodically to automaticallymonitor the fluid sample for compositional changes.

At 214, if the fluid is not the correct type of fluid, for example, ifthe expected fluid is a whole blood sample, while measured sample has adifferent resistivity that matches blood plasma, fluid can be ejectedfrom the reservoir.

At 216, the reservoir can be re-filled from an external supply. To dothis, the microprocessor 168 can send a signal to the external supply torelease a sample into a microfluidic channel for delivery into the fluidreservoir 126.

In parallel with identification of the fluid in the reservoir, a thermalactuation method 206 can be executed to monitor the fluid temperatureand, in response to changes in the temperature, clear the reservoir 126.

First, the fluid temperature is sensed using the method 160 describedabove to measure resistivity. At step 172 of the method 160, instead ofinterpreting the resistivity as a particular type of fluid, theresistivity can be correlated to a fluid temperature.

At 218, it is determined whether or not the fluid temperature exceeds amaximum or a minimum threshold temperature. If the threshold is notexceeded, the method 160 can be repeated continuously or periodically toautomatically monitor the fluid sample for changes. It is noted thatheat dissipated by the reference device 108 can be a source of thermalexcitation of the fluid under test within the reservoir 126.

At 214, if the temperature of the fluid sample is not within a desiredrange, some or all of the fluid can be ejected from the reservoir 126.

At 216, the reservoir 126 can be re-filled from an external supply.

Additionally or alternatively, a volume actuation method 208 can beexecuted to adjust the volume of fluid in the reservoir 126.

First, the fluid volume is measured using the method 180 describedabove.

At 220, it is determined whether or not the volume of the fluid presentin the reservoir 126 exceeds a volume setpoint. If the setpoint is notexceeded, the method 180 can be repeated continuously or periodically toautomatically monitor the fluid volume for changes.

At 214, if the volume of the fluid sample exceeds the setpoint, some orall of the fluid can be ejected from the reservoir 126. The referencedevice 108 can be pulsed so as to effectively trigger athermally-induced fluid ejection at regular intervals.

At 216, the reservoir 126 can be refilled with fresh fluid from anexternal supply. Alternatively, a user can be alerted to supply a freshfluid sample instead of feeding the reservoir from an external supply.

By executing one or more of the methods 202, 204, and 206, it ispossible to monitor fluid samples for quality, or to perform anelectrolytic verification analysis of the fluid sample prior toperforming other biological or chemical testing. Such monitoring canalso detect errors in sample preparation so as to reduce the number offalse negative results obtained by subsequent biochemical testing usingan incorrect sample, or an insufficient volume of the sample. Based onthe results of monitoring, if the fluid sample fails to meet standardsand is ejected, a message can be displayed or transmitted to a user tocommunicate the status of the sample. Furthermore, monitor data can berecorded in the memory 169 for subsequent statistical analysis. Inaddition, a fluid sensor that contains a reservoir and an ejectionmechanism can be re-usable as opposed to disposable.

So, in addition to the improved form factor that a sensor reservoiroffers over that of an impregnated sensor strip, it has beendemonstrated herein that an integrated resistive sensor can besignificantly more useful than an integrated capacitive sensor. Ingeneral, more information can be gathered by the resistive sensor and,furthermore, such additional information can be used to controloperation of the sensor itself.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary, to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A microelectronic fluid detector, comprising: an integrated voltage divider circuit, including: a first resistor including a reference resistive element, a first resistance of the reference resistive element being known; and a second resistor including a fluid reservoir, the second resistor being electrically coupled to the first resistor, a second resistance of the fluid reservoir varying with changes in a fluid present in the fluid reservoir.
 2. The microelectronic fluid detector of claim 1 wherein the changes in the fluid include changes in a volume of fluid present in the integral fluid reservoir.
 3. The microelectronic fluid detector of claim 1 wherein the changes in the fluid include changes in a type of fluid present in the integral fluid reservoir.
 4. A device, comprising: a first electrode and a second electrode; an insulator including a first portion that spaces the first electrode from the second electrode; and a reservoir having sidewalls formed by the first electrode, the second electrode, and the first portion of the insulator.
 5. The device of claim 4, further comprising: a resistive layer, the first electrode and the second electrode being on the resistive layer.
 6. The device of claim 5 wherein the insulator includes a second portion that spaces the first electrode and the second electrode from the resistive layer.
 7. The device of claim 6 further comprising: vias extending through the second portion of the insulator, the vias electrically coupling the first electrode and the second electrode to the resistive layer.
 8. A method, comprising: applying an electrical signal to a pair of electrodes partially surrounding a reservoir, the pair of electrodes being electrically coupled to a reference resistor; measuring a response signal between the pair of electrodes; and determining a characteristic of a fluid in the reservoir based on the electrical signal, a resistance of the reference resistor, and the response signal.
 9. The method of claim 8 wherein the electrical signal is a current signal, and the response signal is a voltage signal.
 10. The method of claim 8 wherein the electrical signal is a voltage signal, and the response signal is a current signal.
 11. The method of claim 8 wherein the pair of electrodes is spaced from each other by an insulator, and the reservoir is formed by the pair of electrodes and the insulator.
 12. The method of claim 8, further comprising: determining a resistance of the fluid based on the electrical signal, a resistance of the reference resistor, and the response signal.
 13. The method of claim 12, further comprising: determining a volume of the fluid based on the resistance of the fluid and a size of the reservoir.
 14. The method of claim 12, further comprising: identifying the fluid based on the resistance of the fluid.
 15. The method of claim 8, further comprising: determining whether the fluid is in the reservoir based on the response signal.
 16. The method of claim 8, further comprising: ejecting the fluid from the reservoir based on the characteristic of a fluid.
 17. The method of claim 8, further comprising: adding fluid in to the reservoir from an external source based on the characteristic of a fluid.
 18. The method of claim 8, further comprising: determining a temperature of the fluid; and ejecting the fluid from the reservoir based on the temperature of the fluid. 